1. Field of the Invention
The present invention relates generally to a method for manufacturing a gate-rotor and gate-rotor support, and more specifically to a method for cutting precision tooth profiles in a cylindrical gate-rotor or gate-rotor support blank with a multi-axis numerically controlled milling machine.
2. Description of the Prior Art
Recent developments in shipboard operation systems require an efficient high pressure compressor. A device having a constant compressed air flow at about 3,000 psi is desired. One device generally suitable for shipboard operations is a positive displacement type machine known as a single screw mechanism. The single-screw mechanism can be made to operate as a compressor, an expansion machine, a pump, a hydraulic motor, or the like.
The primary components of a single screw mechanism are a mainrotor, a gate-rotor with a gate-rotor support, and a mainrotor housing. The mainrotor, provided with at least one thread, is driven and spins about its center axis. The gate-rotor is generally cylindrical, having at least one gear tooth which meshes with the mainrotor threads and is thereby driven by the mainrotor. The gate-rotor is generally backed by a metal gate-rotor support which follows and supports each gate-rotor tooth in the mainrotor thread. The mainrotor housing is fitted in close proximity to the crests of the mainrotor and is provided with at least one port leading to a suction plenum, and at least one additional part leading to a discharge plenum.
The general operation of a single screw gas compressor is as follows: Gas is drawn into the mainrotor thread from the suction plenum; When the thread is filled with gas, a gate-rotor tooth rotates into position and, in co-operation with the mainrotor housing, closes off the thread to form a compression chamber; As the mainrotor turns, the gate-rotor tooth proceeds through the thread, reduces the compression chamber volume, and thereby compresses the gas; When the desired gas pressure is achieved, the edge of the rotating mainrotor thread uncovers a discharge port in the mainrotor housing and the compressed gas is expelled into the discharge plenum.
High pressure compressors require a precision cut gate-rotor. An accurate mesh between the mainrotor thread and the gate-rotor tooth is a critical requirement for an efficient high pressure compressor. The gate-rotor tooth must accurately mesh with the thread because it forms one constraining boundary of the compression chamber. If the gate-rotor tooth profile does not accurately mesh with the mainrotor thread, the gas leaks from the compression chamber, and the volumetric efficiency of the compressor is reduced. An inaccurate mesh also causes friction and wear, reducing the mechanical efficiency of the compressor.
High pressure compressors also require a precision cut gate-rotor support. The gate-rotor support provides structural support to the gate-rotor teeth, reducing gate-rotor tooth deflection and damage. Gate-rotor tooth deflection and damage are caused by friction and compression chamber pressure forces. Friction and compression chamber forces will bend, twist, and/or break an inadequately supported gate-rotor tooth. The gate-rotor support tooth profile is not critical for low pressure applications, as it must simply fit behind a gate-rotor tooth and travel through the mainrotor thread without interfering. The gate-rotor support tooth profile is critical for high pressure applications, as it must also counteract both the greater frictional wear forces and the greater internal pressure forces. The high pressure devices presently being developed require the gate-rotor support to extend to its maximum width and thickness, becoming a metal extension of the gate-rotor. At these high pressures, slight inadequacies in tooth support increase deflection, structural fatigue, and damage, thereby reducing the efficiency of the compressor. These high pressure gate-rotor supports must therefore be manufactured with the same tooth profile accuracy as the gate-rotors they support.
The sealing surfaces of a gate-rotor tooth form complex shapes. The cylindrical gate-rotor tooth is generally trapezoidal, however the flanks of the tooth are formed by complex angles. The gate-rotor seals in a cylindrical plane, however each tooth has a finite thickness inside and outside of the sealing plane. Therefore each tooth flank must have relief angles to facilitate efficient travel through the mainrotor thread. The tooth flanks generally comprise at least two relief angles, or flank angles, which intersect to form the complex sealing lines of the tooth. Zimmern, in U.S. Pat. No. 4,321,022, discloses tooth profiles having at least three skewed surfaces (and the resultant 2 sealing lines) on each flank. Furthermore, each flank angle varies in size from the base to the apex of the tooth. This variation is necessary to achieve an accurate mesh with the mainrotor thread throughout the length of the compression chamber.
The U.S. Navy is presently developing a grinding method for manufacturing precision gate-rotors for high pressure compressors. Commercially available gate-rotor supports are manufactured by casting. As gate-rotor supports are generally composed of high strength metals, manufacturing gate-rotor supports by a grinding method would be inefficient. This is particularly true for the thick gate-rotor supports of a high pressure compressor.
Precision cut gate-rotors and gate-rotor supports are essential for the efficient high pressure compressor desired. For these and other reasons, a need exists for a method for accurately cutting the complex flank angles of these precision, high pressure gate-rotor and gate-rotor support tooth profiles.